Influence of Cooling Conditions on Long-Period Stacking-Ordered Phase Evolution and Corrosion Behavior of As-Cast Resoloy^®

Abstract

This study focuses on the influence of cooling conditions on the long-period stacking-ordered (LPSO) phase evolution and corrosion behavior of as-cast Resoloy®, a bioresorbable Mg-Dy-based alloy. Metallographic and corrosive tests are used to monitor the changes in the properties of this material. The corrosion behavior is investigated by potentiodynamic polarisation. Permanent mold chill casted ingots are wire-eroded to cylindrical platelets. The eroded platelets are solution heat treated over three different time periods. Cooling is performed in two different ways: quenching in water and cooling in air at ambient temperature. The as-cast condition shows a homogeneous fine-grained microstructure. Grains become larger with increasing heat treatment duration and slow cooling leads to additional grain growth. Furthermore, cooling in air leads to faint lamellar LPSO structures, which develop from bulk LPSO structures during the cooling process. The corrosion rate of the cooled platelets increases with increasing grain size. When the lamellar LPSO structures are uniformly distributed over the entire grain, the corrosion starts at the matrix between the LPSO lamellae and stops at them. Heat treatment at 500 °C reduces the normal potential difference between matrix and secondary phase and thus weakens the galvanic corrosion.

Keywords

• Mg alloy
• Resoloy^®
• bioresorbable stent
• heat treatment
• metallography
• LPSO phase